Detection of benzene and similar volatile organic compounds (VOC) is of high importance for safety and process control in chemical, petrochemical, steel and other manufacturing industries, as well as for minimizing environment pollution with these harmful gases. Benzene (C6H6) is a highly flammable, toxic, human carcinogen, organic hydrocarbon. It is widely used as an intermediate in processes leading to plastics, nylon, lubricants, coke, fertilizers, detergents, etc. In recent years, in many regions, including US and EU, benzene has replaced lead in gasoline composition. Due to its increased harmful potential, severe regulations have been imposed against its industrial use. In EU, gas can contain maximum 1% benzene by volume, while in US the upper limit is 0.62%. Monitoring benzene concentration is a vital requirement for the personal protection of people working in oil and gas storage and transportation, oil refineries, petrochemical industry. At concentration levels higher than 10,000 ppm, benzene can be lethal, while repeated exposures at much lower levels can lead to cancer, heart and brain failures, and endocrine diseases. The level over which benzene becomes harmful is currently set at the threshold limit value (TLV) of 0.5 ppm.
Currently, benzene sensing is performed by employing several techniques: multi-gas monitors, metal-oxides (MOx) based chemo-resistors, electrochemical detectors, fixed or portable gas chromatographs, single gas (colorimetric) detection tubes, and/or photoionization detectors (PIDs). A combination of the last two technologies leads to Ultra RAE3000, a portable benzene and compound-specific VOC monitor commercialized by Honeywell's RAE Systems. Ultra RAE3000 employs a PID, a low energy UV lamp and pre-filter tubes. Honeywell top-solution has an accuracy of +/−10%.
Other sensors that are commercially available for such industrial applications, as well as breath alcohol portable detectors, include a thick film of SnO2 deposited on ceramic substrate, which is heated on the other side by a platinum heater. Even if this sensor is recommended not only for domestic applications, but also for portable applications, it is consuming about 660 mW for heating the substrate to the optimum sensing temperature and reading the detector response. Such a level of power consumption is determining a frequent battery replacement in portable applications, which may raise safety issues in the field operation.
In addition, the above noted sensors are detecting these VOC's gases only at relatively high concentrations, above 50 ppm, while the present requirements for benzene in the ambient are as follow: the threshold limit value (TLV) is 0.5 ppm, the short term exposure limit (STEL) is 2.5 ppm, while the immediately dangerous to health and life (IDHL) level is 500 ppm. Therefore, in safety applications, it is useful to detect much lower gas concentrations and then give an alarm and take an early stage action against any hazardous situation. Therefore, there is a strong motivation for increasing the sensitivity of the existing commercial sensors, as well as decreasing power consumption of VOC sensors so that an electric power much below 100 mW to be used and concentrations much below 50 ppm to be detected for VOC gases.
It is already largely accepted by the business and scientific community that the use of nanostructured sensing materials is increasing the sensitivity due to its material architecture and it is allowing the reduction of the power consumption, due to their large specific area and increased porosity, which are thus increasing the number of active sensing sites, while their surface energy is high enough for the sensing reactions to take place without too much thermal energy added from outside.